The present invention relates to fluid catalytic cracking and the removal of solids from a synthetic liquid hydrocarbon stream derived from hydrocarbon-containing solids.
Numerous processes are well known in the art for the production of synthetic crudes from hydrocarbon-containing solids, for example, coal liquefaction, coal carbonization and shale retorting processes.
One problem encountered in these processes is that of the presence of finely divided solids in the liquid product. Typically the solids range in size from about 0.5 to 50 microns. Coal, for example, can be converted to a valuable product by subjecting coal to solvent extraction to produce a mixture of coal extract and undissolved coal residue. The organic matter is taken into solution by either hydrogen-donor solvent, for example tetralin, or a coal-derived (process-derived) solvent in the presence of hydrogen or a synthesis gas.
Attempts to provide an effective process for converting hydrocarbon-containing solids, particularly coal, to petroleum-like products have generally not been successful as a result of the difficulties encountered in efficiently and effectively separating the insoluble solids from the crude product. In prior art processes, the insoluble residue is typically separated from the liquid product by filtration, centrifuges, hydrocyclones, settling or vapor stripping.
Another problem encountered with raw synthetic crudes is that high-molecular-weight hydrocarbons boiling above 800.degree. F and typically above 1000.degree. F are formed. These high-molecular-weight products tend to further increase the difficulty in separating the finely divided solids from the synthetic crude.
Yet another problem encountered with synthetic crudes is that the crude appears to be unstable and has a tendency to form additional high-molecular-weight polymeric materials further complicating the removal of solids if the synthetic crude is not immediately processed.
It is well known in the art that heavy petroleum fractions can be converted into lighter, more valuable fractions by cracking processes. Thermal cracking accomplishes conversion of high-molecular-weight compounds to lower-molecular-weight compounds by using heat only, while catalytic cracking provides a more selective conversion at lower temperatures. Fixed-bed, fluid-bed and moving-bed catalytic cracking processes are all well known in the art.
In fluid catalytic cracking, the cracking catalyst is maintained in a fluidized state and the feed enters the bottom of the reaction zone and cracks as it passes through the reactor. Carbonaceous material formed during cracking, usually referred to as "coke", is deposited on the catalyst surfaces, thus reducing its activity. It is therefore necessary to regenerate the catalyst so that it can be used again, and this is accomplished by burning off the carbon deposits. Spent catalyst is continuously drawn off from the reactor and passed to a catalyst regeneration zone where the coke deposits are burned off.
Typically, fluid catalytic cracking conditions include a temperature in the range of 850.degree. to 1050.degree. F, a weight hourly space velocity of 1 to 2, a catalyst to oil ratio of 8 to 10, and a severity factor of 5 to 8, where the severity factor is equal to the catalyst oil ratio divided by the weight hourly space velocity. Typically the object of fluid catalytic cracking is to produce as much gasoline stock as possible, that is, material boiling below 430.degree.. Generally process conditions are maintained to provide a per-pass conversion of 50 to 80% of the feed to 430.degree. F- material. Typically, petroleum feedstocks fed to a fluid catalytic cracker are first subjected to a distillation to remove solid contaminants which tend to deactivate the catalyst. Generally feedstocks fed to the fluid catalytic cracker must have high hydrogen/carbon atomic ratios greater than about 1.5 in order that coke deposits do not accumulate too rapidly.
The catalytic cracking of liquids derived from hydrocarbon-containing solids is known in the art, as shown, for example, in U.S. Pat. Nos. 3,700,586 and 3,652,446. However, in these prior art processes either solids have already been removed from the feed material or the feed material has undergone a prior hydroprocessing step to increase the hydrogen content of the feed material to prevent excessive coking in the catalytic cracker or to produce conventional catalytic cracker products, particularly gasoline boiling below 430.degree. F.